Ask the Experts

It turns out there are more PQ experts out there than we thought. This month Frank Waterer, staff engineer, Square D, joins our resident expert, Mark McGranaghan, vice president of consulting services, EPRI-PEAC, to address the issues of step-down transformers and the 2002 NEC's rules on short-circuit ratings.

Mark McGranahan, EPRI-PEAC and Frank Waterer, Square D | Aug 01, 2003

Q. We have a client in California that has a 480V power distribution system with a transformer stepping down to 208V. The ground-neutral on the 480V is fine. At the transformer, we measured different amounts of current on the three different wires feeding from the transformer secondary and a potential difference between the ground that was bonded with the three neutral wires. Each 208V panel in the building has a neutral carrying current as high as one of the phases. The grounds in the panels have some current, but not as much as the neutral. What are the possible causes?

McGranaghan's answer: The 480/208V transformer is a separately-derived source for the 120/208V loads supplied from this transformer. This means the neutral and ground at the secondary of the transformer should be bonded at one location only, usually the transformer secondary, but it could be the main panel supplied by the transformer. With the neutral and ground bonded and tied to the overall building grounding electrode system — building steel — at this location, there should be no voltage between neutral and ground. At individual panels supplied from this transformer, the neutral and ground shouldn't be tied together.

It's normal for current to be in the neutral conductors, especially if the loads supplied are single-phase and include electronic equipment. In fact, the neutral current can often be higher than the phase currents due to the addition of the third harmonic components in the neutral (Fig. 1 and 2). This can result in a significant voltage between the neutral and ground at these panels, depending on the magnitude of the current and the impedance of the neutral circuit in which the current is flowing. This voltage can be in the range of 5V to 10V for high levels of electronic load. Facilities can use a zig-zag transformer or another isolation transformer at these remote panels to limit the flow of neutral currents and the resulting neutral-to-ground voltage if it's a problem.

Your case doesn't seem to fit this normal scenario. You mentioned that there are unbalanced currents and unbalanced voltages on the three phases supplied from the main 208V panel. This is normal if the circuit is supplying single-phase loads. You also mention that there are three neutral wires, indicating that maybe each phase has its own neutral. This is one strategy to avoid overloading the neutrals with the third harmonic current mentioned above. The neutral current can't be any higher than the phase current since each phase has its own neutral. The disadvantage of this approach is that each neutral sees all of the phase current. You have nothing to gain from the cancellation of the fundamental frequency currents that would normally occur in a common (shared) neutral. This can result in higher neutral voltage drops than you would find with a common neutral.

When you say that each 208V panel has a neutral that's as high as one of the legs, I assume you're talking about current magnitude. As mentioned above, this isn't unusual for a shared neutral and it's true by definition if there's a neutral for each phase.

The ground conductors in the 208V panels shouldn't be carrying load current. If there's any significant current — more than leakage current that might be associated with capacitance and surge suppressors — then there must be a neutral-to-ground connection down line from the panel. You should find any such connections and make sure there aren't extra neutral-to-ground bonds. The ground currents can result in interference with computer networks and magnetic fields that cause computer screen problems. This condition may also be an important safety concern if there is ever a fault.

Q. The 2002 NEC requires a TVSS to have a short-circuit rating (SCCR) greater than or equal to the point of installation. My question relates to the interpretation of this new Code requirement. Suppose I have a 3,000A switchboard that's rated at 100kAIC and uses 65 kAIC breakers (via a series rating) to feed each of the branch feeders. If the TVSS is installed into one of these 65kAIC breakers, does it have to be rated at 65kAIC or 100kAIC? The same question would apply to a residential load center that has a main breaker with a 22kAIC rating and 10kAIC breakers to feed the loads. If the TVSS is fed via a 10kAIC breaker in the panel, must the fault current rating of the TVSS be 10kAIC or 22kAIC? In other words, how do series ratings of breakers affect the installation of a TVSS, per Art. 285?

Waterer's response: The answer to your first question is that in this application, the specific TVSS unit employed within the 3,000A switchboard must have an ISC (short-circuit current rating) of 100kA and not 65kA if the available ISC is 100kA. The 65kA rating for the circuit breakers is reportedly associated with a series rating, which can only be applied to a specific combination of selected circuit breakers after extensive testing by an authorized listing laboratory. Since a TVSS unit wouldn't qualify for or with a series rating, the specific TVSS unit associated with the question above must have an ISC rating of 100kA, or equal to or greater than the point on the system where the electrical connection for the TVSS unit will be made.

It's important to note that although the 3,000A switchboard may be constructed and braced for an ISC rating of 100kA, that doesn't necessarily mean that the available ISC from the local electrical energy provider at the specific installation is 100kA. The electrical equipment manufacturer or provider may need to contact the local electrical service provider for more information on the maximum available short circuit current at any specific location.

In response to your second question and in summary, there isn't an applicable series rating for any TVSS unit. Sec. 285.6 of the 2002 NEC addresses the ISC rating issue very clearly, stating “the TVSS shall be marked with a short circuit current rating and shall not be installed at a point on the system where the available fault current is in excess of that rating.” The reference to “point on the system” isn't associated with series ratings or the coordinated opening of two or more circuit breakers. The reference to “point on the system” where a TVSS unit is electrically connected is only associated with maximum available short circuit current at that specific point.